Allow priorities to be associated to Trackers.

We use different kinds of trackers (Issue Calls, Bugs, etc). The priorities for Issue Calls is contractual so they have to be a given set (Level 1, Level 2, and Level 3) and they mean specific things to the Issue Call tracker. The bug and other trackers do not use those priorities. Currently there is no way to define which priorities are available in which Tracker so there are priorities in each tracker that are not used and should not be used.

Since each tracker is potentially for a specific group of people and may be client contract specific it would be nice to choose which priorities are displayed per tracker.

p.s. these trackers are within the same project so they cannot be solely project dependent.